chore: refactor parameter parsing in ListIdentities and disallow combining filters

[alnr]

The code for ListIdentities has gotten out of hand. The string-based query building is not sustainable. @aeneasr and I feel we have no choice for now but to disable combining filters (like GET /admin/identities?organization_id=abc&credential_identifier=xyz).

This will allow us to build a simple switch into the ListIdentities handler to go to optimized implementations for each filter in the future and in cloud.

[codecov]

Codecov Report

Attention: Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.58%. Comparing base (8cbb5bd) to head (de69e47).

Files with missing lines	Patch %	Lines
identity/handler.go	95.12%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4244      +/-   ##
==========================================
- Coverage   79.10%   78.58%   -0.53%     
==========================================
  Files         381      380       -1     
  Lines       28012    27275     -737     
==========================================
- Hits        22159    21434     -725     
- Misses       4209     4221      +12     
+ Partials     1644     1620      -24     

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